Amorphization-induced strong localization of electronic states inCsPbBr3andCsPbCl3studied by optical absorption measurements

Optical absorption spectra of amorphous $\mathrm{CsPb}{X}_3$ films $(X=\mathrm{B}\mathrm{r},\mathrm{C}\mathrm{l})$ are characterized by two Gaussian bands near the fundamental edge, with the optical energy gap largely blueshifted and the absorption intensity strongly reduced as compared with the crystalline films. The peak energies of the bands are close to those of the $A$ and $C$ bands of Pb-doped alkali halides. The spectral features are discussed in terms of a molecular orbital theory based on a quasicomplex ${\mathrm{Pb}}^{2+}{(X}^{-}{)}_6$ model similar to the complex model for the doped alkali halides. It is shown that not only ${\mathrm{Pb}}^{2+}6s$ and $6p$ extended states near the band edges but also $X^{-}\hspace{0.5em}p$ states contributing to upper valence bands are localized by amorphization. The transitions from the localized ${\mathrm{Pb}}^{2+}6s$ to $6p$ states produce the spin-orbit allowed ${}^3{P}_1$ and dipole allowed ${}^1{P}_1$ states responsible for the two Gaussians. The localized $X^{-}p$ states lie deeper in energy than the localized ${\mathrm{Pb}}^{2+}6s$ state and only contribute to higher-energy absorption above the Gaussian bands, giving the reason for the reduced absorption near the fundamental edge. The blueshift of the optical energy gap is attributed to the disappearance of $k$ dispersions for these one-electron states.